Laser systems are widely used in portable applications, including military applications. For example, laser rangefinders and target designators may be carried by soldiers to be used during combat operations. Because such portable laser systems are often used outdoors and in harsh environmental conditions, the laser systems typically are required to operate over broad temperature ranges (e.g., −40° C. to +70° C.). Because such portable laser systems are often carried by soldiers traveling on foot, the weight of such systems is desirably minimized.
Portable laser systems typically use flashlamps to pump the active laser media. The electrical-to-optical power conversion efficiency (termed wallplug efficiency) of flashlamp pumped laser systems is typically low, for example from 1 to 3%. Because of the low wallplug efficiency of flashlamp pumped portable laser systems, a large percentage of the weight of such a laser system (e.g., approximately 20% for target designators) may be devoted to the batteries used to power the system. Spare batteries are typically also carried by users of such flashlamp pumped portable laser systems. As a result, a soldier may carry over 40 pounds of batteries for use with a typical target designator.
Laser diode bars, typically arranged into diode arrays comprising several diode bars, have been developed and used to pump solid state lasers. Because of the ability to match the spectral output of a laser diode array to the peak absorption wavelength of the laser medium, the wallplug efficiency of diode pumped laser systems is typically much greater than the wallplug efficiency of flashlamp pumped laser systems. For example, a flashlamp pumped Nd:YAG (neodymium doped, yttrium aluminum garnet) laser capable of producing 100 millijoules of pulsed output energy may have a typical wallplug efficiency of 3%. A similar diode pumped Nd:YAG laser may have a typical wallplug efficiency of greater than 15%. The increased efficiency of diode pumping typically reduces the required prime power of a diode pumped laser.
A typical laser medium may have one main absorption peak, such that pump light at the frequency corresponding to the main absorption peak will be absorbed more readily by the laser medium than pump light at a different wavelength. The absorption spectrum of a typical Nd:YAG laser medium is illustrated in FIG. 1. A main absorption peak 10 corresponds to approximately 808 nm, such that a diode array emitting light with a wavelength of 808 nm would typically be used to pump a laser using such an Nd:YAG medium. (The wavelength of the output of a diode array is specified at a defined operating temperature, for example at 25 degrees Celsius (° C.), and varies to higher or lower wavelengths if the operating temperature differs from the defined operating temperature.) A laser medium may have more than one absorption peak other than the main absorption peak, such as absorption peak 12 of FIG. 1 (three of which are highlighted in FIG. 1). Pump light at a wavelength corresponding with such an absorption peak (other than the main absorption peak) may also be readily absorbed by the medium, although not as efficiently as light that corresponds to the main absorption peak 10. A typical diode pumped laser would therefore use a diode array with a wavelength corresponding to the main absorption peak of the laser medium to maximize absorption.
However, the wavelength of the light emitted from a diode array typically changes as its temperature changes. By way of example, this wavelength shift may be 0.3 nanometers per degree Celsius (nm/° C.) for one diode array. Since the output spectral width of a typical diode array is approximately 3 to 6 nanometers, temperature changes of the diode array of more than a few degrees C. from the optimum temperature (i.e., the temperature at which the wavelength of the output of the diode array matches the peak absorption wavelength of the laser medium) will cause the spectral output of the diode array to shift away from the peak absorption wavelength such that the absorption of the pump energy by the laser medium will greatly decrease, and therefore the efficiency of the laser system will also decrease greatly. Because of the efficiency decrease that may be caused by temperature changes, diode pumped lasers typically require careful thermal management to minimize temperature changes of the diode array. For example, a typical Nd:YAG laser may be pumped by diode arrays that emit light at a wavelength of 808 nm at a desired operating temperature. Such a laser may require that the temperature of the diode arrays be maintained within approximately plus or minus 5° C. to maintain the emitted wavelength at or close enough to the peak absorption wavelength of the laser medium to maintain laser efficiency within approximately 5 to 10% of peak efficiency, i.e., the peak absorption wavelength lies within the output spectral width of the diode array. As such, complex thermal management equipment, such as diode array coolers and heaters, are typically used to maintain the optimum temperature of the diode array in a diode pumped laser system. However, such thermal management equipment increases the size and weight of the laser system, which is particularly undesirable for portable laser systems. Such equipment also increases the complexity of the laser system, which may be undesirable in combat situations. The cost of adding such thermal management equipment increases the cost of diode pumped laser systems. Additionally, the electrical power needed to operate the heaters and coolers decreases the overall laser system efficiency and therefore requires more batteries, which in turn further increases the weight and/or decreases the lifetime of the system.
As such, there is a need for a portable laser system with increased efficiency to reduce the battery requirements while maintaining the ability to operate across a wide temperature range.